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Unpredictability will remain a feature of highly individualised photoablative laser refractive treatments until more reliable means become available to evaluate patients’ preoperative vision, their cornea’s biomechanical properties and the likely effects of corneal epithelium healing on the refractive outcome, said Michael Mrochen PhD, Zurich, Switzerland.

“I think we have good tools to address certain patients. With normal myopic eyes, moderate amounts of astigmatism, we get excellent outcomes. But if we go to more complex cases, I think we have a big gap in terms of predictability and giving good visual outcomes,” he told the XXXII Congress of the ESCRS in London.

Examples of the types of patients who may require complex treatments include those with good Snellen visual acuity but poor quality of vision following LASIK because of higher-order aberrations, older patients who want spectacle independence by mono-vision, and those with residual astigmatism after cataract surgery.

Also requiring such individualised treatments are those undergoing presbyopic corrections and those undergoing topography-guided treatment for therapeutic reasons.

“The factors that need improvement in such cases include the predictability of refractive outcomes, the ability to measure and correct aberrations quality of vision or enhance the depth of field. And finally we have some biological limitations, epithelium healing responses and biomechanical factors,” added Dr Mrochen.


Irregular corneas

Planning treatment in complex cases is difficult because the preoperative refraction measurements are less dependable in such eyes.

As an illustration, he cited a study showing that the mean repeatability of subjective refraction was similar to the predictability of LASIK in normal eyes, with around 90-95 per cent of measurements being repeatable within half a dioptre, compared to only 44 per cent in eyes with keratoconus.

“Corneal irregularity can definitely influence the predictability of the refraction. But how can we improve our outcome if the gold standard that we use is subjective refraction?

“Our gold standard is actually hiding the possible improvements. So the reproducibility of the subjective refraction is relevant if we want to improve our outcomes in the future,” Dr Mrochen noted.

In addition, the precision of treatment is not always matched by the same precision in outcome, owing to the unpredictable effects of corneal healing.

The optically perfect corneal shape is in some cases not the biologically optimum shape in structural terms. As a result the corneal epithelium will fill in the slight structural irregularities in the stromal surface that can be present after a finely-tuned wavefront-guided or wavefront-guided treatment.

That explains why, although such treatments are effective in reducing large amounts of aberration, they are less effective for smaller aberrations. It also shows why inducing small amounts of spherical aberration with sub-micron precision, as in the case of presbyopic LASIK, is inherently unpredictable using current approaches.



He noted that inducing a certain amount of aberration can increase depth of focus by 1.0D to 2.0D, although with some compromise in visual quality.

The treatments most of the companies are using for presbyopic correction nowadays try to leave the eye with 0.3 microns to 0.6 microns of spherical aberration over a 6.0mm pupil.

He suggested that preoperative optical coherence tomography (OCT) imaging of the epithelium might make the epithelial effects on postoperative corneal surface curvature more predictable in such cases.

“Epithelial smoothing is masking existing stromal irregularities and might smooth an attempted correction. It can have a lot of relevance in presbyopia correction or for therapeutic applications if we were to do a topography guided treatment,” Dr Mrochen said.

Finally, the cornea's biomechanics also lend an amount of uncertainty to the finer adjustments needed for wavefront-based correction of spherical aberration.

A range of technologies for measuring the cornea’s viscoelastic properties such as Brillouin scattering, dynamic OCT or dynamic Scheimpflug techniques are coming to the market, which may make it possible to take the biomechanical effects of laser refractive surgery into consideration when planning laser ablations.

In addition, collagen cross-linking might be used as a way of actually controlling the biomechanical effects of surgery in order to reduce variability in refractive outcomes.

Validation of preoperative measurements and treatment plans based on postoperative outcomes is essential to achieving any improvements in the efficacy of the procedures.

The advent of electronic recording and analysis of treatment outcome data could facilitate the creation of nomograms individualised to each surgeon, to further improve the outcomes.

“To improve our outcomes, I think we need to reconsider the gold standard for evaluation and improve the diagnostics with high resolution OCT for the epithelium and biomechanical measurements by technologies such as Brillouin scattering.

“I think we have to make individualised assumptions on the corneal response based on those measurements. Last but not least, as clinicians running a refractive surgery centre, you also have to keep in mind that you have competitors and you want to find a way to differentiate yourself from your competitors based on the technology you’re using and based on the outcomes you’re achieving,” said Dr Mrochen.


Michael Mrochen:

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